Pulling rolls are used in the manufacture of sheet glass to apply tension to the ribbon of glass from which the sheets are formed and thus control the nominal sheet thickness. For example, in the overflow downdraw fusion process (see Dockerty, U.S. Pat. Nos. 3,338,696 and 3,682,609), pulling rolls are placed downstream of the tip or root of the fusion pipe and are used to adjust the rate at which the formed ribbon of glass leaves the pipe and thus determine the nominal thickness of the finished sheet.
Pulling rolls are preferably designed to contact the glass ribbon at its outer edges, specifically, in regions just inboard of the thickened beads which exist at the very edges of the ribbon. A preferred construction for such rolls employs discs of a heat resistant material, which are mounted on a driven shaft. Examples of this construction can be found in Moore, U.S. Pat. No. 3,334,010, Asaumi et al., U.S. Pat. No. 4,533,581, and Hart et al., U.S. Pat. No. 5,989,170.
A successful pulling roll needs to meet a number of conflicting criteria. First, the roll needs to be able to withstand the high temperatures associated with newly formed glass for substantial periods of time. The longer a roll can last in such an environment the better, since roll replacement reduces the amount of finished glass a given machine can produce and thus increases the ultimate cost of the glass.
Second, the roll must be able to produce enough pulling force to control the glass' thickness. In order not to damage the central portion of the ribbon that becomes the usable finished glass, the roll can only contact the ribbon over a limited area at its edges. Thus, the required pulling forces must be generated using only this area. However, the forces applied to the glass cannot be too large since this can create surface damage which can propagate into the usable central portion of the ribbon. Accordingly, the roll must achieve a balance between applying too little and too much force to the edge regions of the glass.
Third, the pulling roll must not give off excessive amounts of particles which can adhere to the glass and form surface defects (known as “onclusions”). For glass that is to be used in such demanding applications as substrates for flat panel displays, onclusions must be kept to very low levels since each onclusion will typically represent a defective region of the finished product (e.g., one or more defective pixels). Because of the hot environment in which pulling rolls operate, finding materials which can apply sufficient pulling forces to a glass ribbon and yet not give off particles when hot is a difficult challenge.
The existing pulling rolls have not been able to fully satisfy these competing criteria of long life, controlled force application, and low contamination. The present invention addresses these shortcomings in the art and provides improved pulling rolls which achieve higher levels of performance than existing pulling rolls.